Alkanes and AlkenesTopic 10.2 and 10.3

Alkanes have low reactivity

bond enthalpies are relatively strong348 kJ mol-1 to break a C-C bond412 kJ mol-1 to break a C-H bond

low polarity only readily undergo combustion reactions

with oxygen (very flammable) and substitution reactions with halogens in UV light

Reactions of Alkanes: Combustion hydrocarbons (only contain C & H) complete combustion

alkanes burn in an excess supply of oxygen to form carbon dioxide and water: example:

C8H18 (g) + 12 ½ O2 (g) → 8 CO2 (g) + 9 H2O (l)

exothermic (-∆H) incomplete combustion

if oxygen supply is limited, the gas carbon monoxide and carbon is formed C8H18 (l) + O2 (g) → C (s) + CO (g) + CO2 (g) + H2O (l)(notice left over carbon (black soot) and dangerous CO)

Reactions of Alkanes (methane and ethane) with Halogens (Cl and Br) alkanes do not react with halogens in the

dark at room temperature, but will react in the presence of sunlight (UV) a substitution reaction will occur where

some or all of the hydrogens will be replaced with a halogen C2H6 (g) + Br2 (g) → C2H5Br (l) + HBr (g) Cl2 + CH4 CH3Cl + HCl

this happens by a process know as free radical substitution that happens in 3 steps1. Initiation2. Propagation3. Termination

initiation initiated by UV light breaking a chlorine molecule into two free radicals by a process called homolytical fission (* = unpaired electron)

Cl2 Cl* + Cl* propagation

keeps the chain going (radical in reactants and products)

CH4  +  Cl* CH3*  +  HCl

CH3*  +  Cl2 CH3Cl  +  Cl*

termination this removes free radicals (*) from the system without

replacing them by new ones

Cl* + Cl* Cl2 CH3

* + Cl * CH3Cl CH3

* + CH3* CH3CH3

each resulting atom receives one unpaired electron, known as free radicals that have lots of energy

each resulting atom receives one unpaired

electron, known as free radicals

Reactions of Alkenes (Topic 10.3.1):the general mechanism alkenes react with many substances to

form a new substance catalysts, acids or other substances may

be required to complete the reaction:C2H4 + XY → CH2XCH2Y

process occurs by breaking the double bond.

Reactions of Alkenes (10.3.2) : with hydrogen

alkenes react with hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC:

C2H4 + H2 → CH3CH3

Reactions of Alkenes: with halogens

alkenes react readily with chlorine or bromine to create a dihalogenalkane (general name)

C2H4 + 2 Cl2 → CH2Cl CH2Cl

H

H H

H

+HH

ClHH

ClCl Cl

1,2-dichloroethane

Reactions of Alkenes (10.3.2)with hydrogen halides

alkenes react readily with hydrogen halides to create a halogenalkanesC2H4 + HBr → CH3CH2Br

H

H H

H

+HH

HHH

ClH Cl

Reactions of Alkenes with water alkenes do not react readily with water. if sulfuric acid is used as a catalyst, an

alcohol to be created remember that H2O can be dissociated into H+ and

OH-

C2H4 + H2O → CH3CH2OH

H

H H

H

+ OHH

a c idO

H

H

H H

H

H

Distinguish between alkanes and alkenes using bromine water

bromine water (a red liquid) tests for unsaturated hydrocarbons (alkenes) alkanes → stay yellow/orange

no reaction alkenes → turn clear / colourless

because of reaction with unsaturated hydrocarbon

http://www.youtube.com/watch?v=NjIuBvod2eM

Reactions of Alkenes: Polymerization naming polymers

put “poly-” in front of the name of the monomer

there are 3 polymerization mechanisms that you need to be familiar with:

1. polyethene2. polychloroethene3. polypropene

Polyethene monomer: ethene CH2=CH2

undergoes additions reactions with itself to make a chain

n CH2=CH2 [-CH2-CH2-]n

Polychloroethene each chloroethene contains 1 chlorine therefore when the chloroethene

molecules polymerize, every other carbon will bond to 1 chlorine

this is PVC

Polypropene

+ =

Teflon non-stick pans

Outline the economic importance of the reactions of alkenes (10.3.5) making margarine

hydrogenation (addition of H) of vegetable oils (alkenes)

making ethanol ethene + water

drink it or use as fuel

H

H H

H

+ OHH

a c idO

H

H

H H

H

H

making plastics Teflon

tetrafluoroethene

PVCpolychloroethen